This invention relates to methods for preparing cubic boron nitride sintered body and cubic boron nitride as well as a method for preparing boron nitride for use in the same.
In recent years, much attention has been paid particularly to the sintered body of boron nitride as one of fine ceramics. With regard to kinds of boron nitride stable under a high pressure, cubic boron nitride and wurtzite boron nitride are known. Among them, sintered body of cubic boron nitride has been spotlighted of late because of being excellent in hardness and abrasion resistance, and new applications of cubic boron nitride to materials for tools are now considered to be promising. Further, since having an extremely high thermal conductivity and electrical resistance together, sintered body of cubic boron nitride is greatly expected as a heat sink material in high-density integrated circuits.
The techniques for manufacturing a cubic boron nitride sintered body stable under a high pressure, can be classified under two methods, an indirect method and a direct method. The indirect method comprises using a cubic boron nitride as the starting material which was prepared by the steps of converting a mixture of hexagonal boron nitride and a catalyst into cubic boron nitride through a reaction at high pressure and a high temperature, removing the used catalyst and the remaining unreacted hexagonal boron nitride from the system by chemical means, sintering the thus prepared cubic boron nitride under an ultra-high pressure and at a high temperature (e.g., 5 GPa or more and 1200.degree. C. or higher), alternatively further adding thereto a cubic boron nitride powder, a binder mainly including TiN or Al.sub.2 O.sub.3 and, if necessary, a metal and/or a metallic compound, and again carrying out a high-pressure and high-temperature reaction to prepare the cubic boron nitride sintered body. On the other hand, the direct method comprises using a boron nitride other than the cubic boron nitride as the starting material subjecting a mixture of the boron nitride and a catalyst to a high-pressure and high-temperature reaction, whereby the boron nitride is converted into cubic boron nitride and simultaneously the cubic boron nitride sintered body can be prepared. The prepared cubic boron nitride sintered body is composed of the direct bonding of cubic boron nitride grains.
Of these two methods, the former utilizes the two high-pressure and high-temperature treatment steps which involve great costs, but the latter requires such a costly treatment step only once. Therefore, it seems that the latter is more suitable than the former. This can be easily understandable because in the direct method, the conversion rate of hexagonal boron nitride into cubic boron nitride will be below 100% in most cases, and the remaining unreacted hexagonal boron nitride will noticeably impair properties and a performance of the prepared sintered body. Therefore, there are factors favoring synthesizing, by the direct method a cubic boron nitride sintered body stable under a high pressure.
On the other hand, for manufacturing pure cubic boron nitride stable under a high-pressure, there is a direct method which can be carried out under a hydrostatic pressure without using any catalyst and a catalytic method of using a specific catalyst material. The boron nitride, stable under a high pressure, which can be prepared in accordance with the direct method is a mixture of cubic boron nitride and wurtzite boron nitride, and thus pure cubic boron nitride cannot be manufactured by means of the direct method. On the other hand, with regard to boron nitride stable under a high pressure which was prepared by the catalyt method as disclosed in Japanese Patent Publications No. 14/1963 and No. 17838/1977,there is an advantage that they can be prepared even at a lower pressure and temperature than in the case of the direct method as well as they includes no wurtzite boron nitride. However, in these catalytic manufacturing methods, hexagonal boron nitride is used as a starting material, and there is a problem that conversion rate of the hexagonal boron nitride into cubic boron nitride is low.
The present inventors have found that the factors having the greatest influence on the conversion of the boron nitride starting material into cubic boron nitride the a crystallinity index and content of oxygen of the boron nitride starting material. On the basis of this finding, in Japanese Patent Publications No. 37200/1981 and No. 34429/19, methods are disclosed for manufacturing a cubic boron nitride sintered body and cubic boron nitride which comprise treating a mixture of hexagonal boron nitride and aluminum nitride as a catalyst at a high temperature and a high pressure under a condition of 2% by volume or less of an oxygen content of the mixture.
The present inventors have further studied the relation between the crystallizability of hexagonal boron nitride and the conversion rate, and as a result it has been confirmed that the boron nitride which has smaller crystallinity, i.e., amorphous boron nitride, has the highest conversion rate into cubic boron nitride (see Funtai Funmatsu Yakin Kyokai, Outline Booklet at Spring Congress Lecture, 1983, pp.96-97).
Moreover, the present, inventors' research has been extended to a group of various compounds usable as materials for amorphous boron nitride, and as consequence, they have accomplished this invention.